"The novel is not so much a sexual and social farce but a gentle analysis of human interaction as it examines the vagueness and vagaries of existence. (...) Never heavy in its discussion of art, friendship, and work -- Plainsong reads like a Jean-Luc Godard movie scripted by Samuel Beckett with added jokes by Richard Brautigan and Charles Bukowski." - Steve Finbow, The Japan Times

"(T)he narrator's cat obsession is quite suffocating (we're talking page after page of cat observation, dialogue about cats, philosophizing about the life of a cat) and a poor substitute for a plot, though late in the book Akira instigates an outing that lends the plainspoken work some shape. Must love cats." - Publishers Weekly

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Plainsong begins with the narrator ready to take a significant step in his adult life -- moving in together with his girlfriend, in a large new apartment -- but his course is derailed even before he manages to get that far: his girlfriend dumps him "just before the big day", leaving him with the huge apartment all to himself.
With two bedrooms, a living room, a dining room, and kitchen it's "larger than anywhere I'd lived before".
He's reached that stage in life where "weekends suddenly get harder to fill", because almost all his friends have begun to settle down; having fallen just short of that himself, he now finds himself still somewhat adrift.
The narrator has and takes up a few small preoccupations and diversions: betting on horse races is one, engaging with the local stray cat(s) another.
He also doesn't remain alone: his apartment slowly fills up with passing and then remaining guests, a group of slightly younger barely-adults even more adrift than he is (the narrator at least has a secure job, while they don't seem to care too much how they get by).
He also reaches out to a former classmate, Yumiko, calling her initially to ask her advice about cats after not having spoken to her for several years, and then remaining in touch with her; about the same age as him, Yumiko already has a child (though she isn't married).
One of those that moves in with him is a would-be filmmaker.
He admits:

I started out wanting to write a novel.
But you can't write a novel where nothing happens.
You can't just depict the simple passage of time in writing.

Plainsong isn't exactly an attempt to prove that wrong, but with its limited action and incidents the focus is decidedly on the slow drift of everyday life (specifically of those who haven't embraced the rat-race, or the salary-man- or even traditional-domestic-lifestyle).
Would-be filmmaker Gonta's ambitions are realized here:

I want to show people that the life we live has nothing to do with the stories you see in movies or novels, where everything is simplified and dramatic and exciting.
Our lives are our stories.

And, yes, those lives are pretty slow-moving and uneventful.
An amusing exchange has Yumiko remind the narrator about his attempts to write book reviews for a surfing magazine after he graduated from college (suggesting, incidentally, that he was about as clue- and ambition-less as the kids he's putting up in his apartment now).
His effort to write something about Haruki Murakami was dismissed because: "our readership wasn't interested in hard-to-read stuff like that", and even reviews of comic books and photo collections were dismissed as not of interest; he wound up writing about: "Australian surfing magazines and stuff".
Here, already, he was confronted with a cultural shift, trying his best to promote worthy books but coming up against an audience indifferent to anything but the most base and simple -- the ultimate slacker (non-)culture.
While not too obviously critical of his new flatmates, the narrator does now also catch himself sounding:

just like the guys who are always droning on about how things were "back in our day"

As his surfing magazine days suggest, things weren't that different back then either -- save that the attitude seems to have spread (while he remains at its periphery, unable (or unwilling) to entirely escape it).
In case the symbolism hasn't been obvious enough, Hosaka begins to really hammer it home by the end.
So, for example, a beach excursion leads to observations such as:

It feels like we've been marooned or something, floating here like this.

They see themselves for what they are: flotsam.
Plainsong is a novel of younger generations of Japanese: the narrator and Yumiko, "born ten years or so after the war, for whom the Tokyo Olympics, the Osaka Expo, and the Winter Olympics of Sapporo had been the events that punctuated the early stages of our lives", and the generation some ten years younger than them, represented by the kids who move in with the narrator, just coming of age in the 1980s.
Almost unmentioned is the novel is the explosion of the Japanese economy in that period -- the incredible boom and eventual bust of the 1980s.
Neither the narrator nor his flatmates have -- or seem to want to have -- much to do with that.
Instead, they practice various forms of escapism: from the location of the apartment -- out in the boondocks (one reason it is affordable) -- to the hobbies (horse racing, the cat-obsessions) to the excursion to the seaside (and then out into the sea itself) everything sets them apart, rather than involving them.
Bustling, vibrant Japan is barely even a backdrop to their lives.
It makes for an interesting slice of Japanese life, and attempt to capture a generation or two, at a specific time in Japanese history.
An agreeably placid fiction of the times.